Nitrous oxide gas, (N2O), is produced as a by-product in a synthesis of hydroxylamine. Hydroxylamine may be used to form a caprolactam that can be used to form nylon 6. Nylon 6 is an important polymer that is used throughout the world in carpets, apparel, upholstery, auto parts, and in many other products.
Typically, hydroxylamine is formed from partially hydrogenating nitric oxide gas (NO) with hydrogen gas (H2) in an aqueous medium including nitrogen gas (N2) as an inert gas. Mixing the NO with the H2 and the N2 forms hydroxylamine in addition to a variety of by-products. The by-products include N2O. The N2O, in addition to unused N2, H2, and NO along with water vapor (H2O), may be evacuated from the hydroxylamine as a moist gas mixture that may be disposed of or recycled. The gas mixture may also include contaminants including methane, carbon dioxide (CO2), and carbon monoxide (CO). The gas mixture may be recycled along with H2 and NO to reduce overall consumption of H2 and NO.
However, if the gas mixture and the H2 and NO are recycled, a level of the N2O can accumulate in the gas mixture to dangerous levels. If the level of N2O in the gas mixture becomes too high, the gas mixture becomes flammable and unsafe to handle. If the level of N2O reaches a predetermined upper limit, the gas mixture is discarded, resulting in loss of H2, and NO remaining in the gas mixture. Removal of N2O from the gas mixture would preferably prevent premature disposal of the gas mixture along with H2 and NO thereby allowing recycling of the H2 and NO and reducing overall consumption.
Adsorption can provide a more efficient and economic means for separating gases than use of cryogenic distillation, absorption, or membrane-based systems. Adsorption can be used to separate a gas from the gas mixture that includes at least two gases that have different adsorption characteristics toward an adsorbent material.
Typically, adsorption generally includes a synchronized cycling of a pressure or temperature of one or a plurality of adsorbent beds. The adsorbent bed includes the adsorbent material that preferentially adsorbs one or more gases present in the gas mixture. Pressure swing adsorption (PSA) generally includes a series of steps that change the pressure and/or flow direction of the gas mixture to achieve separation of a preferred gas from a series of non-preferred gases. The number and nature of the steps involved in PSA may vary based on separation objectives.
More specifically, PSA typically includes flowing the gas mixture from a feed inlet of the adsorbent bed to a discharge end of the adsorbent bed, at an adsorption pressure. A gas in the gas mixture that has strong adsorption characteristics preferentially adsorbs onto the adsorbent material, while a gas with weak adsorption characteristics continues to flow through the adsorbent bed. As such, the gas mixture flowing out of the discharge end of the adsorbent bed is substantially depleted of the gas preferentially adsorbed onto the adsorbent material.
As the gas with the strong adsorption characteristics flows from the feed inlet to the discharge end, the gas with the strong adsorption characteristics accumulates on the adsorbent material closest to the feed inlet. As the accumulation proceeds, the adsorbent material closest to the feed inlet becomes saturated first, followed by saturation of the entire adsorbent material in the direction of flow of the gas mixture.
Eventually, once the entire adsorbent material becomes saturated, the gas with the strong adsorption characteristics breaks through the discharge end. Usually, the flow of the gas mixture is stopped before breakthrough occurs. Once the flow of the gas mixture is stopped, the gas with the weak adsorption characteristics may be removed without excessively desorbing the gas adsorbed onto the adsorbent material. After the adsorbent pressure is reduced, the adsorbent bed is further depressurized and the gas adsorbed onto the adsorbent material is removed from the bed. The adsorbent bed is then purged and re-pressurized to the adsorption pressure with the gas having the weak adsorption characteristics. Once this occurs, the adsorbent bed may be reused.
The adsorbent materials that may be used with PSA are dependent on the gas to be adsorbed, the gas mixture itself, and other factors that are well known to those skilled in the art. In general, suitable adsorbent materials include zeolite molecular sieves, silica gel, activated carbon, and activated alumina. For certain applications, specialized adsorbent materials can be used.
The efficiency of PSA depends on a variety of parameters including pressures, temperatures, volumes, and flow rates of gases in PSA systems, time of the synchronized pressure cycling, types, sizes, and shapes of the adsorbent materials, dimensions of the adsorbent beds, and compositions of feed, product, purge, and other intermediate gas streams. Variations in these parameters can influence the cost and productivity of the PSA systems.
Conventional PSA systems and cycles can remove N2O from the gas mixture. However, conventional PSA methods known in the art potentially form flammable gas mixtures of N2O, H2, and NO and many adsorbents do not exhibit a high selectivity for the N2O as compared to the H2O in the gas mixture. As such, there remains an opportunity to develop a PSA method that effectively separates N2O from H2O in the gas mixture such that N2O of high purity can be removed from the gas mixture using a minimum number of PSA systems and the N2, H2, and NO can be efficiently recycled.